The present invention relates to a surge absorber provided in a power supply unit or the like of an electronic equipment susceptible to surge voltage which serves to avoid problems or malfunctions of the electronic equipment caused by surge voltage. More particularly, the present invention relates to a surge absorber using a semiconductor-type surge absorbing element.
A semiconductor-type surge absorbing element is typically represented by a silicon two-terminal thyristor element having a PNPNP or NPNPN junction structure. When a surge voltage higher than a breakover voltage (V bo) is applied between a pair of terminal electrodes and the thyristor is ignited, an electrical short-circuiting takes place between the terminal electrodes and the surge absorbing element absorbs the surge. When a power supply is connected between the electrodes of this surge absorbing element and an electric current of over holding current (Ih) is supplied, the on-state is maintained by the surge. When the current exceeds the current capacity of the element, breakage of the element occurs. To prevent this phenomenon from occurring, the conventional surge absorber 3 has a structure, as shown in FIG. 6, in which a resistor 2 is connected in series to a semi-conductor-type surge absorbing element 1.
As shown in FIG. 7, this surge absorber 3 is connected in parallel with an electronic equipment unit 5 in the upstream portion of the equipment to cope with a surge coming into the electronic equipment 5 from a commercial AC power supply 4.
This semiconductor-type-surge absorbing element 1 has properties as shown in FIG. 8. More specifically, it is necessary to use a breakover voltage (V bo) of the surge absorbing element 1 higher than the peak value (Vp) of the AC source voltage employed. At the same time, it should be appropriately set taking into account the response characteristic to surge and the surge withstanding voltage of the electronic equipment 5. The peak one cycle surge current (I tsm) which is a kind of current capacity of the semiconductor-type surge absorbing element 1 should be as large as possible.
On the other hand, the resistance value (Rs) of the resistor 2 should be set so as to satisfy the formula (1), relative to the peak value (Vp) source voltage employed: EQU Rs&gt;Vp/I tsm (1)
Accordingly, after ignition of the semiconductor-type surge absorbing element 1 suffering from the incoming surge, the current flowing from the power supply is limited by the resistor 2, and it is thus possible to inhibit the surge voltage without breakage of the surge absorbing element 1.
By selecting the characteristic values of the element in accordance with a concept similar to that described above, this practice is applicable also to a commercial power line of a voltage of AC 200 V, for example, other than AC 100 V.
Furthermore, selection of the characteristic values of an element in this manner permits its application also to a DC power line. In such an application, however, a characteristic value of the semiconductor-type surge absorbing element that must be considered is the holding current (Ih). More particularly, it is desirable to use a semiconductor-type surge absorbing element satisfying the following formula (2) relative to the DC source voltage (V dc) which is adopted and the internal impedance (Rz) of the power supply: EQU Ih&gt;V dc/Rz (2)
However in the case of the following formula (3) EQU Ih&gt;V dc/Rz (3)
it suffices to set a value so as to satisfy the following formula (4) relative to the resistance value (Rs) of the resistor 2: EQU Rs&gt;(V dc/Ih)-Rz (4)
In a surge absorber 3 as shown in FIG. 6, it has been the conventional practice to use a wire wound resistor, a metal film resistor, or a metal oxide film resistor, for example, as the resistor 2, and independently attach a semiconductor-type surge absorbing element 1 and the resistor 2 on the substrate as two separate parts.
The surge absorber thus obtained by separate attachment of the semi-conductor-type surge absorbing element and the resistor on the substrate requires many manhours for mounting on the substrate. Also, it occupies a large area on the substrate, making it impossible to achieve a compact surge absorber.
Since a small-sized wire wound resistor, metal film resistor or metal oxide film resistor leads to a small surge current capacity and cannot be utilized as a part of a surge absorber, it is necessary to use a large-sized resistor. This makes it even more difficult to downsize the surge absorber.